In fact the errors are not with Sliding Notes but with what content you put inside. W3C is pretty anal on containment rules, and WordPress has it’s own constrains. (I’ve written elsewhere about this.) Sliding Notes do the best they can to correctly render ANY content that you put inside, such that it looks good on screen, for the actual visitors on which your website success depends – even if that inserting that content into a note would violate W3C containment rules.

Bottom line: This “issue” is not an issue with Sliding Notes, and can’t be resolved on Sliding Notes’ end. If W3C purity is imperative to you, and you can’t simplify your note content, I recommend you try to find another solution.

Well, of course I tried putting inside just one word, but the 9 errors remains.
Your plug-in, obviously, works perfectly even with those errors.
Maybe it’s a conflict with one or more the wordpress plug-ins in my site.